I'd appreciate any advice on troubleshooting this issue. I have an old natural gas furnace made by Central Environmental Systems in York, PA. Model# P3UGD08N08001A, Serial# ENSM374304. I've been operating this unit since I purchased my home about 3 years ago. Each winter when I go to fire the heat up, the pilot light will not stay lit. I've gotten pretty efficient at changing the thermocouple after watching the service tech the first time. Anyway, this fall has been the same story, with a little twist. I installed the new thermocouple and fired the furnace up for a few days of cool weather. Weather got warmer, and I shut it down again for 2 or 3 weeks. Getting cooler again, and as I expected, the pilot light won't stay lit - with a basically new tc. I replaced it with another (2nd of the season), but my elementary troubleshooting skills have now failed me. I know I need a new, more efficient unit, but that's not really in the budget right now.

If it's not the thermocouple, what are the next steps? Air filter is good, breaker is good, power is good. I've got a multimeter, and I'm not afraid to use it. I'm fairly mechanically inclined, but being a gas furnace makes me second guess trying to fix it myself. However, the anticipation of the service bill may outweigh that.

It appears the thermocouple is not telling the gas valve that there is a pilot flame. There are two parts to this puzzle. One is the thermocouple location in the pilot flame and the other is the electrical circuit.

Check the pilot flame and make sure it's a good strong flame with a blue core. Check that the flame doesn't get blown around by drafts. Make sure the thermocouple is positioned a little above the blue part of the flame. There is an adjustment screw on the gas valve to increase the pilot flame if necessary.

Now the electrical. Since the thermocouple is sending an electric signal to the gas valve (unless this is a very old and different gas valve) you need to make sure the furnace is very well grounded. Turn off the power at the breaker and check all 120 volt wiring from the breaker to the furnace. Make sire all connections to the furnace control board are secure. I would also check the main house grounding system.

Last possibility is the gas valve. Here is a good - *.achrnews DOT com/articles/troubleshooting-the-standing-pilot-safety-system

I'm fairly certain the gas valve is going bad. A lite tap on the valve with a hammer did the trick for now. I lit the pilot and while holding the button down, gently tapped on the gas valve. Whatever was stuck, or hung up, freed and when I let go of the button the pilot stayed lit. Problem solved. For now.